The invention relates to a method for determining the condition of a measuring probe, and to a measuring probe and a single-stage or multi-stage process system which are compatible with the practice of the method.
The control of industrial processes, for example in the chemical and pharmaceutical industries, in the textile industry, in the food and beverage industries, in the processing of paper and cellulose, or in the fields of water processing and waste water treatment, is based on the measurement of process variables that are determined by means of suitable measuring probes or sensors.
Reference [1], “Process-Analytical Systems Solutions for the Brewery”, a company publication of Mettler-Toledo GmbH, CH-8902 Urdorf, Article No. 52 900 309, with a printing date of September 2003, describes how suitable measuring probes are used in the individual stages of a process chain of a brewery (i.e., in the water processing stage; the brew house; the fermentation and storage cellar; the filtration-, carbonization- and filling stages; as well as the waste water treatment stage) to determine the conductivity, the amount of dissolved oxygen, the pH value, the CO2 value, and the turbidity of the process liquid.
The signals of the measuring probes are transmitted by way of measurement converters (also referred to as transmitters) and in some cases through couplers and a common data bus to a process computer or a central computer which evaluates the measuring signals and controls the process.
It should be noted that with the high level of automation that exists in the fields of food technology and biotechnology, the production systems are in most cases closed systems with permanently installed pipe conduits in which numerous measuring probes are used. Surfaces that are not adequately disinfected and sterilized present health risks. Contamination deposits favor the breeding of undesirable microorganisms as they offer ideal conditions for growth in the form of nutrients and a suitable temperature. Furthermore, microorganisms in deposits are more difficult to deactivate. Completely cleaned surfaces are therefore a fundamental prerequisite for disinfecting and sterilizing a process system. The cleaning of process systems has therefore become a complex procedure and its technical realization has become a demanding task (see [2], Dr.-Ing. Karl Welchner, “The Top Commandment, A Reproducible Cleaning Process for Process Systems as a Core Quality Criterion” (Part 1) Pharma+Food 2/2000).
For a trouble-free process control, it is further of particular importance to monitor the condition of the individual measuring probes as their properties normally change after an extended period of operation.
Reference [3], DE 102 09 318 A1, describes that the wear on a measuring probe manifests itself through a change of one or more parameters that are relevant to the correct functioning of the probe. It is proposed to determine the wear-dependent remaining operating time of a pH- or oxygen probe from its calibration parameters, specifically the zero point, the slope, the impedance and the settling time.
In [4], DE 101 00 239 A1, a method is described for determining the remaining operating time of a potentiometric measuring probe which contains an electrolyte as well as a primary and a secondary reference element that are arranged in such a manner that an electrolyte deficiency which advances from an opening of the measuring probe can be detected by means of a secondary reference element before the electrolyte deficiency reaches the primary reference element which interfaces with a voltage potential that is to be measured. After the difference between the potentials measured at the primary and secondary reference elements has exceeded a given limit value, the remaining operating time can be determined and indicated.
The aforementioned reference [4] further describes how contaminations can occur on measuring probes that are used for the monitoring of chemical or microbiological processes, whereby errors can be introduced in the measuring result. Contaminating deposits will therefore have to be removed not only in the process system or its individual components but also from the measuring probes in order to ensure correct measuring results as well as an uncompromised sanitary state of the process system. Because of the large number of measuring probes used in such systems, they are normally not uninstalled for the cleaning, but are cleaned and sterilized with a CIP- (Cleaning In Place) or SIP- (Sterilizing In Place) process. The CIP processes prevent any bacterial growth, and the pipe conduit is freed of contaminating particles after the end of the process. The process consists of pumping either a cleaning/disinfecting liquid or simply hot water through the pipe conduit system.
CIP- compatible measuring probes, conductivity sensors, pH sensors, O2 sensors, CO2 sensors, and turbidity sensors as well as suitable armatures and process system connectors are disclosed in [1]. Also shown in reference [1] are control modules (EasyClean) which offer a solution to realize automated cleaning and calibrating systems.
The interval time between calibrations of an electrochemical measuring probe can be determined for example through a procedure as described in [5], DE 101 41 408 A1. A basic time interval is prescribed for a defined basic range of values of at least one measurement parameter of a medium to be measured, which parameter is relevant to the adaptation of the calibration interval and is monitored during the operation of the measuring probe. Subsequently, the deviation of the detected measuring parameter values from the defined basic range of values is determined, and the calibration time interval is adapted dependent on the detected deviation.
Thus, according to references [3] and [4], the condition of the measuring probes is determined from changes in the properties of the measuring probes. Reference [5] teaches a method of determining the time intervals within which the measuring probes have to be recalibrated in order to compensate for changes that may have occurred.
As described above, these procedures involve considerable effort and expense and may in some cases require a compatible design of the measuring probes.
It is also known that measuring probes should be exchanged after a certain number of CIP- or SIP cycles.
As disclosed in reference [6], Jochen Endress, “Non-stop in Action”, Pharma+Food 1/2002, page 36, the allowable cumulative CIP exposure times are different for individual measuring probes. It is further possible that CIP- or SIP processes are not performed in all parts of a process system. Also, the CIP- or SIP processes in the individual parts of the system may differ for example in regard to the temperature being used, so that wear conditions of different severity occur in the different parts. It is also possible that different cleaning processes are performed sequentially in a process system stage. As an example, each time a number n of CIP processes have been performed, an SIP process is executed. Thus, considering all possible variations of the processes running in a system, in particular the CIP- and SIP processes, the administration of the measuring probes of an entire process system becomes very complicated and expensive. On the other hand, information that is simply based on an overall count of how many CIP- and SIP processes have been initiated is not precisely representative of the condition of the measuring probes used in the system.
All of the foregoing documents mentioned are hereby incorporate by reference in their entireties.